Question: $\sum\limits_{n=0}^{2 }{{(-n)}}=$
Solution: What is the question asking for? $\sum$ tells us to find the sum. The question is asking for the sum of the values of $-n$ from $n = 0$ to $n = 2$. Evaluating $\begin{aligned} \sum\limits_{n=0}^{2 }{({-n})}&= (-0) + (-1) + (-2) \\\\ &= 0 + (-1) + (-2) \\\\ &= -3\end{aligned}$ The answer $\sum\limits_{n=0}^{2 }{({-n})}=-3$